1. Field of the Invention
The present invention relates to liquid electrophotographic imaging systems, and in particular to a developing apparatus having a developing roller for supplying a latent image substrate with liquid developer and a control method thereof.
2. Description of the Related Art
There has been known a liquid electrophotography using a liquid developer composed of charged toner particles dispersed in a dielectric fluid. An electrostatic latent image formed on a latent image substrate is developed by adhering toner particles to surfaces of the latent image substrate due to electrophoresis. As a liquid developer supplying system, it is generally known that a thin layer of liquid developer is formed on a cylindrical roller or a belt to continuously supply the liquid developer to a developing region of the latent image substrate. In the case of a developing roller, the developing roller is rotated to continuously supply a predetermined amount of liquid developer to the surface of the latent image substrate. In this case, the developing roller also serves as an opposed electrode.
The liquid developer supplied from the developing roller to the latent image substrate forms a meniscus on the developing region of the latent image substrate. In this case, the amount of toner developed on the latent image depends on the amount of liquid developer supplied. Therefore, in order to increase the developing speed, the concentration of solid component of liquid developer supplied is increased or the amount of liquid developer supplied is increased by rotating the developing roller more quickly.
After the developing process has been completed, an image portion on the surface of the latent image substrate is normally composed of toner particles and dielectric fluid. This may cause the shape of image to be liquidly lost or disordered depending on the concentration of the toner. To avoid such an undesired phenomenon and perform the transferring process smoothly, a squeezing process is adopted to squeeze only dielectric fluid from the image portion.
In the squeezing process, there is known such a method that dielectric fluid is squeezed by making a rotating roller touch on the latent image substrate under the high pressure. Also, there is known another method that applies an electric field between the roller and the latent image substrate with a gap to drift toner particles toward the latent image substrate and then removes upper dielectric fluid.
There has been disclosed a liquid image forming apparatus having a single roller used as both a developing roller and a squeezing roller in Japanese Patent Application Unexamined Publication No. 6-186859. In this prior art, the roller rotates in the same direction as that of a photoreceptor drum, allowing excess liquid developer to be removed from the surface of the photoreceptor drum. In addition, a liquid reserving plate is provided over the roller such that it makes contact with the roller, preventing liquid developer from going around the roller.
However, in the described-above prior art, developing can be ensured by: supplying the liquid developer in excess of the minimum required for developing: using a higher concentration of liquid developer; excessively increasing the rotation speed of the roller with respect to the photoreceptor drum; or relatively elongating the developing time using a plurality of rollers. As a result, an increasing amount of liquid developer causes the amount of dielectric fluid contained in it to be also increased.
When the amount of dielectric fluid is increased, a lower concentration of liquid developer comes into contact with the developed portion for a relatively long time. Therefore, a solid component ratio in a toner image after the developing process is reduced, where a solid component ratio is defines as a ratio of toner in a developed portion composed of toner and solvent. As a result, there is a high probability that the developed image is disordered and developer is deposited on an area of an image substrate for background, so that a clear image cannot be obtained.
Moreover, depending on the rotation speed of the developing roller, the image itself is disordered in some cases. In addition, as the solid component ratio of toner layer becomes lower, the higher load such as application of pressure or electric field in the squeezing process is required.
In particular, when the developing roller is shared with the squeegee roller and the liquid reserving plate is provided, the developing region can be exposed to a smaller concentration of liquid developer intercepted by the liquid reserving plate. Therefore, the solid component ratio could not be improved. An increased solid component ratio allows a toner image to be prevented from drifting and the transferring process to be performed smoothly, and in addition, the quality of the final image to be improved.
Thus, even in the above-prior art, the excess liquid developer can be removed from the developed toner image on the surface of the photoreceptor drum. However, the problems such as image drifting and a high load required in squeezing could not been solved. Also, an appropriate amount of liquid developer to obtain clear images has not been defined.
The inventors found that a developer supplying condition can be optimized so as to maximize the solid component ratio of a developed toner layer, and thereby the disadvantages of the described-above prior art can be solved.
An object of the present invention is to provide a liquid electrophotographic developing apparatus thereof capable of preventing image drift and reducing the load in the squeezing process.
According to the present invention, a developing apparatus develops an electrostatic latent image on a latent image substrate using a liquid developer in which toner particles are dispersed in a dielectric fluid, wherein the latent image substrate moves at a predetermined velocity in a first direction The developing apparatus includes: a developing roller facing the latent image substrate with a predetermined spacing, for supplying the liquid developer to the latent image substrate while rotating at a peripheral velocity in a second direction, wherein the second direction is opposite to the first direction in a developing region on the latent image substrate, wherein a peripheral velocity ratio of the peripheral velocity of the developing roller to the predetermined velocity of the latent image substrate is determined so as to maximize a solid component ratio of a developed toner layer on the latent image substrate.
A peak peripheral velocity ratio (kt) providing a maximum solid component ratio of a developed toner layer may be represented by
kt=(2mt/Lxcfx81c)+1, 
where mt is a saturated amount of developed toner, L is a length of the predetermined spacing, xcfx81 is a density of the liquid developer, and c is concentration of the liquid developer.
The peripheral velocity ratio (k) may be set to a value ranging from 0.8kt to 1.2kt.
A peak peripheral velocity ratio (kt) providing a maximum solid component ratio of a developed toner layer may be represented by   kt  =            mt      +                                    mt            2                    +                      2            ⁢            mtLc            ⁢                          xe2x80x83                        ⁢            ρ                                      Lc      ⁢              xe2x80x83            ⁢      ρ      
where mt is a saturated amount of developed toner, L is a length of the predetermined spacing, xcfx81 is a density of the liquid developer, and c is concentration of the liquid developer. The peripheral velocity ratio (k) may be set to a value ranging from 0.8kt to 1.2kt.
A glass transition temperature of the toner particles dispersed in the dielectric fluid may be equal to or lower than xe2x88x921xc2x0 C. The solid component ratio of developed toner layer may be 20 wt % or more.
An image forming apparatus includes: a latent image substrate moving at a predetermined velocity in a first direction; a developing roller facing the latent image substrate with a predetermined spacing, for supplying a liquid developer including toner particles to the latent image substrate while rotating at a peripheral velocity in a second direction, wherein the second direction is opposite to the first direction in a developing region on the latent image substrate; a squeezing roller for squeezing the developed toner layer to produce a toner image on the latent image substrate; and a transfer section for transferring the toner image to another medium, wherein a peripheral velocity ratio of the peripheral velocity of the developing roller to the predetermined velocity of the latent image substrate is determined so as to maximize a solid component ratio of a developed toner layer on the latent image substrate.
An image forming apparatus includes: a photoreceptor belt rotating at a predetermined velocity in a first direction; a plurality of developing devices corresponding to different colors, the developing device being arranged along the first direction, each of the developing devices comprising: a developing roller facing the photoreceptor belt with a predetermined spacing, for supplying a liquid developer including color toner particles to the photoreceptor belt while rotating at a peripheral velocity in a second direction, wherein the second direction is opposite to the first direction in a developing region on the photoreceptor belt; and a squeezing roller for squeezing the developed toner layer to produce a color toner image on the photoreceptor belt: and a transfer section for transferring a multicolor toner image obtained by a sequence of the developing devices to another medium, wherein a peripheral velocity ratio of the peripheral velocity of the developing roller to the predetermined velocity of the photoreceptor belt is determined so as to maximize a solid component ratio of a developed toner layer on the photoreceptor belt.